The technique of making small electric components suitable for use in products such as computers and telecommunication equipment has evolved during the last half century into a major branch of industry, producing ever-smaller components. To name an example, the transistor has gone from being a centimetre-sized object in the early days of it's development in the 1940's and 1950's to a sub-micrometer object today.
However, there are still obstacles to be overcome in the field of miniaturisation of electric components. In particular, components that require certain spatial properties, i.e. shape, are still difficult to miniaturise while still retaining optimal electric properties. Such components include inductors, transformers, capacitors etc.
Of course, there have been numerous attempts to produce these types of miniaturised components. For example, three-dimensional micro-machined inductors have been studied by several groups. The geometry of the structures are typically solenoids. Examples of the state of the art include the work presented by J. B. Yoon et al., “Monolithic integration of 3-D electroplated microstructures with unlimited number of levels using planarization with a sacrificial metallic mold”, IEEE MEMS-1999 as well as U.S. Pat. No. 5,793,272, which shows an integrated toroidal inductor. U.S. Pat. No. 5,793,272 describes a toroidal coil produced by a dual-damascene process. A 1.4 nH coil produced by this process achieved a Q value of 40 at 5.8 GHz.
However, all these state of the art integrated inductors for radio frequency application are based on a planar geometry. The limitations of planar integrated coils are several and include that the Q value of the inductor is limited by self-resonance due to the parasitic capacitance of the coil through capacitive coupling to the substrate. Also the ratio of the inductance and series resistance is not optimal. Secondly, the magnetic field of the inductor couples to the surrounding electronics. Hence, interference with other parts of the electronics limits the density of inductive components on the chip.
Moreover, planar inductors with high Q values are large in terms of silicon surface area, an area that cannot be utilised for any other purpose.